(1) Field of the Invention
The present invention relates to a method of producing grain-oriented silicon steel sheets having an easy magnetization axis &lt;001&gt; in the rolling direction.
(2) Description of the Prior Art
Grain-oriented silicon steel sheets are mainly used in iron cores of a transformer and other electric instruments. Recently, it has become an important problem to decrease the electric power loss and to use efficiently the electric power of a transformer and other electric instruments in view of energy saving and resource saving, and grain-oriented silicon steel sheets having more improved magnetic properties have been demanded. As the magnetic properties of grain-oriented silicon steel sheet, which can satisfy the above described demands, there have been required an excitation property of a magnetic induction of B.sub.10 value of at least 1.85 Tesla in the rolling direction under a magnetic field intensity of 1,000 A/m, and a low iron loss of not more than 1.20 W/kg of W.sub.17/50 (iron loss under a magnetic induction of 1.7 Tesla and at an alternate current of 50 Hz). Recently, an excellent grain-oriented silicon steel sheet having a low iron loss of W.sub.17/50 of not more than 1.10 W/kg has been obtained.
In the production of grain-oriented silicon steel sheets having such excellent magnetic properties, it is necessary to develop completely secondary recrystallized grains during the final annealing in the production process of the sheets, and to produce the product steel sheet formed of secondary recrystallized grains having a strong (110)[001] orientation.
In order to develop completely secondary recrystallized grains, it is commonly known that it is indispensable to use an inhibitor which suppresses strongly the normal grain growth of primary recrystallized grains having an undesirable orientation other than the (001)[001] orientation during the secondary recrystallization stage. As the inhibitors, there are generally used fine precipitates of MnS, MnSe, AlN and the like, and the precipitated state of these fine precipitates is controlled mainly in the hot rolling step to develop strongly the inhibiting effect. Recently, the above described fine precipitates are used together with grain boundary segregation elements, such as Sb, Bi, Sn, Pb, Te and the like, to supplement the effect for suppressing the growth of primary recrystallized grains having undesirable orientation and to develop fully the action as an inhibitor.
Further, in order to develop completely secondary recrystallized grains, it is very important not only to use the above described inhibitor, but also to form primary recrystallization texture which can develop predominantly secondary recrystallized grains having (110)[001] orientation in a steel sheet before the final annealing. Such primary recrystallization texture can be obtained only when treating conditions in all process from the hot rolling process to the cold rolling process in the production of grain-oriented silicon steel sheet are properly combined. Particularly, it is important to select properly the final cold rolling reduction rate depending upon the strength of the suppression effect of inhibitor. For example, it is known that, when MnS or MnSe is used as an inhibitor, a proper final cold rolling reduction rate is within the range of 40-80%, and in this case an optimum primary recrystallization texture is formed of strong (110)[001] orientation as a main component and weak {111}&lt;112&gt; orientation as a sub-component.
Recently, there has been developed a method for improving the primary recrystallization texture by utilizing effectively carbon or carbide contained in steel. For example, Japanese Patent Application Publication No. 14,009/63 proposes a method, wherein a hot rolled sheet is very rapidly cooled before the first cold rolling from a temperature of not lower than 790.degree. C. to a temperature of not higher than 540.degree. C., and then kept to a temperature of 310.degree.-480.degree. C. to precipitate lens-shaped carbides having an optical-microscopically visual size (several .mu.m ) in the crystal grains. The resulting relatively large size carbide particles act effectively in order that elongated coarse grains formed during the hot rolling step are divided into small size. That is, the large size carbides have probably an action for reducing coarse grains having (100)[011]-(110)[011] orientations, which are harmful for the development of secondary recrystallized grains, in the initial stage of cold rolling.
Further, there has been recently developed a method, wherein solute C or finely dispersed carbide in crystal grains is utilized during the cold rolling. Japanese Patent Application Publication Nos. 13,846/79 and 29,182/79 disclose a method, wherein a hot rolled sheet containing AlN as an inhibitor is heated to a high temperature and then rapidly cooled, and the annealed steel sheet is subjected to one time of cold rolling at a high cold rolling reduction rate of at least 80%, and further to at least one time of ageing treatment between the cold rolling passes. The above described Japanese patent application publications describe that, in this ageing treatment, it is necessary to keep the steel sheet to a temperature within the range of 50.degree.-350.degree. C. for at least one minute or to a temperature within the range of 300.degree.-600.degree. C. for 1-30 seconds, and further a large number of repeating ageing treatments are effective. However, according to such method, the cold rolling efficiency is very poor, and a high cost is required in the ageing treatment of steel sheet, and therefore the method is not economical. The inventors have disclosed in Japanese Patent Application Publication No. 19,377/81 a method, wherein a combination system of AlN and Sb is used as an inhibitor, and a cooling in an intermediate annealing is carried out such that a steel sheet heated in the intermediate annealing is gradually cooled within the temperature range of 900.degree.-700.degree. C. in 200-2,000 seconds, and then immediately rapidly cooled from 700.degree. C. to a temperature of not higher than 200.degree. C. in 4 minutes, preferably at a very high cooling rate similar to water quenching, in order to exhibit the effect of the combination use of AlN and Sb. However, when it is intended to cool gradually a steel sheet from 900.degree. to 700.degree. C. in 200-2,000 seconds, it is necessary that the cooling zone of a continuous annealing furnace is greatly remodeled to arrange a very long gradual cooling zone, within which the steel sheet is substantially heated and thermally insulated, and further the continuous annealing furnace is operated at a low speed. Therefore, this method is not an economical method due to the low production efficiency of the product steel sheet and the high production cost thereof, and cannot be practically carried out. Moreover, all the above described three methods can develop their effect only when the use of a specifically limited inhibitor of AlN or Aln--Sb is combined with the high final cold rolling reduction rate of at least 80%. The primary recrystallization texture obtained by these methods is formed of very strong {111}&lt;112&gt; orientation as a main component and weak (110)[001] orientation as a sub-component. Therefore, the above described three methods are fundamentally different from a method for developing primary recrystallization texture having strong (110)[001] orientation, and moreover the methods have not been able to be applied to the production of grain-oriented silicon steel sheet by the use of a commonly used inhibitor of MnS or MnSe.
According to Japanese Patent Application Publication No. 3,892/81, which is one of commonly known methods, wherein at least one of MnS and MnSe is used as an inhibitor and carbon contained in a steel is effectively utilized in order to improve the recrystallization texture by carrying out a final cold rolling at a reduction rate suitable for the inhibitor, a steel sheet heated in the intermediate annealing is cooled at a rate of at least 150.degree. C./min within the temperature of 600.degree.-300.degree. C., and the intermediately annealed steel sheet is subjected to an ageing treatment during the final cold rolling. In this method also, it is necessary that the ageing treatment is carried out at a temperature of 100.degree.-400.degree. C. for from 5 seconds to 30 minutes and at least one time of the above described ageing treatment is carried out between cold rolling passes. Therefore, this method is not economic due to the low cold rolling efficiency and the high ageing treatment cost as described above, and a more effective method has hitherto been demanded.
Recently, a continuous casting method is used in place of a conventional ingot making-slabbing method in the production of a slab to be used as a starting material for the production of grain-oriented silicon steel sheets. However, the use of a continuously cast slab increases troubles, which are few cases in the conventional ingot making-slabbing method, in the grain-oriented silicon steel sheet product. That is, when it is intended to obtain fine precipitates of MnS, MnSe, AlN and the like, which are effective as an inhibitor, it is necessary that a slab is heated at a high temperature of not lower than 1,250.degree. C. for a long period of time before the hot rolling to dissociate and to solid solve fully the inhibitor element into the steel, and the cooling step at the hot rolling is controlled to precipitate the inhibitor element having a proper fine size. However, in the continuously cast slab, extraordinarily coarse crystal grains are apt to develop during the high temperature slab heating as described above, and incompletely developed secondary recrystallized texture called as poorly oriented fine grain streaks is formed in the resulting silicon steel sheet due to the extraordinarily coarse slab grains, and the silicon steel sheet is often poor in the magnetic properties.
There have hitherto been proposed several methods in order to prevent the formation of the above-described fine grain streaks and to improve the magnetic properties. For example, Japanese Patent Laid-Open Application No. 119,126/80 discloses a method, wherein a slab is subjected to a recrystallization rolling at a high reduction rate when the slab is hot rolled into a given thickness, that is, the texture of the slab just before the recrystallization rolling is controlled such that .alpha.-phase matrix contains at least 3% of precipitated .gamma.-phase iron, and the slab is subjected to a recrystallization rolling at a high reduction rate of not less than 30% per one pass within the temperature range of 1,230.degree.-960.degree. C. The inventors have proposed in Japanese Patent Application No. 31,510/81 a method, wherein a slab is mixed with a necessary amount of C depending upon the Si content, and not less than a given amount of .gamma.-phase iron is formed within a specifically limited temperature range during the hot rolling, whereby coarse slab grains developed during the high temperature heating are broken to prevent effectively the formation of fine grain streaks in the product.
However, according to the above described method of forming not less than a given amount of .gamma.-phase iron in a slab during its hot rolling, although formation of the fine grain streaks in the product can be prevented, the aimed magnetic properties can be not always obtained, and moreover the prevention of the formation of the fine grain streaks is very unstable, and poorly oriented fine grain texture may be formed all over the product to deteriorate noticeably its magnetic properties. Therefore, this method is still insufficient in the stability of the effect, which is a most important factor in the commercial production of grain-oriented silicon steel sheets.